fnord,

the only thing that might make me think that there weren't three aces out was the pre-flop play...

I also was little surprised you didn't baby-raise on the turn to gain info from player 5, who was clearly representing the straight.

Otherwise, there was no way you could have known. Unless you suspected that ONE of the other two had the sucker straight, in which case you should bet a mid-sized bet to get the call and spilt it with the other ace.